The 727, 518 and 618 all use the same seal.
Make sure the bushing behind the seal is not loose causing the leak or the new seal could also leak.
AND put a little grease in the back of the seal so the spring doesn't pop out when you install it.
I have seen leaks that when inspected I found the spring out of the seal hanging on the output shaft because it popped out when they drove the seal in.
